Signal transduction proteins important in carcinogenesis and cancer progression present attractive targets for the development of novel anticancer therapeutics. The family of proteins, Signal Transducer and Activator of Transcription (STAT), are activated in response to cytokines and growth factors and promote proliferation, survival, and other biological processes (Bromberg, J. Breast Cancer Res., 2000, 2:86-90; Darnell, J. E., Jr. Nat. Rev. Cancer, 2002, 2:740-749; Yu, H. and Jove, R. Nat. Rev. Cancer, 2004, 4:97-105). STATs are activated by phosphorylation of a critical tyrosine residue, which is mediated by growth factor receptor tyrosine kinases, Janus kinases or the Src family kinases. Upon tyrosine phosphorylation, dimers of STATs formed between two phosphorylated monomers translocate to the nucleus, bind to specific DNA-response elements in the promoters of target genes, and induce gene expression. Aberrant activity of one of the family members, Stat3, contributes to carcinogenesis and tumor progression by upregulating gene expression and promoting dysregulated growth, survival, and angiogenesis, and modulating immune responses (Darnell, J. E., Jr. Nat. Rev. Cancer, 2002, 2:740-749; Yu, H. and Jove, R. Nat. Rev. Cancer, 2004, 4:97-105; Bromberg, J. and Darnell, J. E., Jr. Oncogene, 2000, 19:2468-2473; Bowman, T. et al. Oncogene, 2000, 19:2474-2488; Turkson, J. and Jove, R. Oncogene, 2000, 19:6613-6626; Buettner, R. et al. Clin. Cancer Res., 2002, 8:945-954; Turkson, J. Expert Opin Ther Targets, 2004, 8:409-422; Darnell, J. E. Nat. Med., 2005, 11:595-596).
As a critical step in STAT activation (Shuai, K. et al. Cell, 1994, 76:821-828), the dimerization between two STAT monomers presents an attractive target to abolish Stat3 DNA-binding and transcriptional activity and inhibit Stat3 biological functions (Turkson, J. et al. J. Biol. Chem., 2001, 276:45443-45455; Turkson, J. et al. Mol Cancer Ther, 2004, 3:261-269). Stat3 dimerization relies on the reciprocal binding of the SH2 domain of one monomer to the pTyr peptide containing APY*LKT (Ala-Pro-pTyr-Leu-Lys-Thr) (SEQ ID NO: 1) sequence of the other Stat3 monomer. To pursue the development of inhibitors of Stat3 signaling, key structural information gleaned from the X-ray crystal structure of the Stat3β homodimer (Becker, S. et al. Nature, 1998, 394:145-151) was used in the computational modeling and automated docking of small-molecules into the SH2 domain of a Stat3 monomer, relative to the bound native pTyr peptide, in order to identify binders of the Stat3 SH2 domain, and potentially disruptors of Stat3:Stat3 dimers (Shao, H. et al. J Biol. Chem., 2004, 279:18967-18973; Song, H. et al. Proc Natl Acad Sci USA, 2005, 102:4700-4705).